The data on units in structure (also referred to as "type of structure") were obtained from answers to long-form questionnaire Item 34, which was asked on a sample basis at both occupied and vacant housing units. A structure is a separate building that either has open spaces on all sides or is separated from other structures by dividing walls that extend from ground to roof. In determining the number of units in a structure, all housing units, both occupied and vacant, are counted. Stores and office space are excluded. The statistics are presented for the number of housing units in structures of specified type and size, not for the number of residential buildings.

This is a 1-unit structure detached from any other house; that is, with open space on all four sides. Such structures are considered detached even if they have an adjoining shed or garage. A 1-family house that contains a business is considered detached as long as the building has open space on all four sides. Mobile homes to which one or more permanent rooms have been added or built also are included.

This is a 1-unit structure that has one or more walls extending from ground to roof separating it from adjoining structures. In row houses (sometimes called townhouses), double houses, or houses attached to nonresidential structures, each house is a separate, attached structure if the dividing or common wall goes from ground to roof.

Both occupied and vacant mobile homes to which no permanent rooms have been added are counted in this category. Mobile homes used only for business purposes or for extra sleeping space and mobile homes for sale on a dealers lot, at the factory, or in storage are not counted in the housing inventory. In 1990, the category was "mobile home or trailer."

Data on units in structure have been collected since 1940 and on mobile homes and trailers since 1950. In 1970 and 1980, these data were shown only for year-round housing units. A category of "other" was used in 1990, but this category was greatly overstated. It was replaced by "Boat, RV, van, etc." in Census 2000. A similar category, "Boat, tent, van, etc." was used in 1980. In Census 2000, this question was asked on a sample basis. In 1990 and prior to 1980, the unit in structure question was asked on a 100-percent basis. In 1980, data on units at address were collected on a 100-percent basis and data on units in structure were collected on a sample basis. The 1980 data on "units at address" should not be used a proxy for "units in structure" because some multiunit buildings had more than one street address.

A housing unit is vacant if no one is living in it at the time of enumeration, unless its occupants are only temporarily absent. Units temporarily occupied at the time of enumeration entirely by people who have a usual residence elsewhere are also classified as vacant. New units not yet occupied are classified as vacant housing units if construction has reached a point where all exterior windows and doors are installed and final usable floors are in place. Vacant units are excluded from the housing inventory if they are open to the elements; that is, the roof, walls, windows, and/or doors no longer protect the interior from the elements. Also excluded are vacant units with a sign that they are condemned or they are to be demolished.